Server Maintenance Page

Something like this should do it;

when HTTP_REQUEST {
if {[active_members [LB::server pool-name-here]] < 1} {
  HTTP::respond 200 content "Sorry, this server is down for mainenance" noserver Content-Type 
  "text/html" Connection "Close"
  Stop processing the iRule for this event here
  return
  }
}

Perfect. What if I was looking for something more elaborate like a full webpage with a couple of images and CSS formatted text?

For extra formatting, something like this;

when HTTP_REQUEST {
if {[active_members [LB::server pool-name-here]] < 1} {
 HTTP::respond 200 content [subst {
  
  Blocking Page
  
  You got blocked buddy. Try using a valid URI, not [HTTP::uri].
  
  
  } ]
}
}

Or for css and images, using iFiles;

iFiles must be uploaded in advance. Note there is no requirement to 
specify the file extension and thus two files cannot have the same name
index is a html file, some-image is an image file, requested and sent file
names do not need to match

when HTTP_REQUEST {
 if {[active_members [LB::server pool]] < 1} {
  switch [HTTP::uri] {
   "/" { HTTP::respond 200 content [ifile get "index"] }
   "/some-image.jpg" { HTTP::respond 200 content [ifile get "some-image"] }
  }
 }
}

Awesome, thank you.

You're very welcome. Merry Christmas